The Online Positioning User Service (http://www.ngs.noaa.gov/OPUS/) is provided without fees by the National Geodetic Survey. According to the National Geodetic Survey:
“This Online Positioning User Service (OPUS) provides you with simplified access to high-accuracy National Spatial Reference System (NSRS) coordinates: all you need is a clear view of the sky and a survey-grade (preferably dual frequency) Global Positioning System (GPS) receiver.
OPUS processes your GPS data files with the same models and tools which help manage the Nation’s Continuously Operating Reference Station (CORS) network, resulting in coordinates which are both highly accurate and highly consistent with other users. Your computed NSRS position is sent privately via email, and, if you choose, can also be shared publicly via the NGS database.
OPUS is highly automated and requires minimal user input. To use properly, please familiarize yourself with the information provided under the About tab at the OPUS site.